Structural and Instrumental Marxism, how do they differ?

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I've been recently studying the sociological ideas behind Marxism, and read up about Miliband and Poulantzas theories and what not, however I'm pretty lost about how they differ and what they're actually saying. Could anyone direct me towards a website or book which explains this relatively simply? Or simply explain to me, if it's not too long an explanation, exactly how structural and instrumental Marxism differs?

Thanks a lot!